It would perhaps replace the shield factor but not the damage factor. Damage factor is something different. It means, if your worm has dmg factor 2, all your hits make double damage on other worms.

Btw., perhaps what make you think that it would be the same: Each worm can have different damage factor / shield. This is already possible, just not used yet. But it may be that we add some objects (like bonuses) in game which you can select and then you get a better shield or better damage factor.

Also, shield and just more health are also not exactly the same thing. Imagine when you select a health-bonus. A health-bonus of 50 health points means somethings different in case you have a shield-factor of 2 or if you have just started with health 200.